The haploid human genome is estimated to contain 4,000 near-full length LINE-1 (L1Hs) elements. L1Hs elements belong to the non-LTR (or poly A) class of transposable elements. Insertional mutagenesis mediated by the transposition of L1Hs has been shown to result in the development of human genetic disease and may contribute to the development of human cancers. The focus of our studies has been L1Hs transcription and its regulation. Our goals are to understand the mechanism of L1Hs transposition, it's regulation, it's frequency, and it's impact on the human genome and human disease. The consensus L1Hs element, derived from both genomic and cDNA sequences is 6.2 kb long and contains (from left to right) a 900 bp non-translatable region (UTR), two open reading frames of approximately 1 and 4 kbp separated by a short (approximately 30 bp) spacer sequence, a 200 bp 3' UTR, and a variable-length A-rich sequence. L1Hs contains no long terminal repeats. The currently favored model for the mechanism of L1Hs transposition involves (i) synthesis of full-length polyadenylated L1Hs RNA directed by regulatory sequences located within the element, (ii) translation of this RNA into proteins, one or more of which are required for transposition, (iii) reverse transcription of the RNA by an L1Hs-encoded enzyme, and (iv) insertion of the cDNA into staggered chromosomal breaks. Evidence in support of this model has been accumulating over the past several years. Previous work in this and other laboratories demonstrated that full-length, polyadenylated L1Hs RNA is transcribed in the human teratocarcinoma cell line NTera2D1. The regulatory region responsible for promoting and regulating the transcription of abundant L1Hs RNA in a cell- specific manner was identified within the L1Hs 5' UTR. Polypeptides translated from L1Hs RNAs have recently been identified. These include the product of ORF-1, a leucine-zipper-motif containing phosphoprotein called p40, and the product of ORF-2 which has been demonstrated to encode a functional reverse transcriptase when expressed in Saccharomyces cerevisiae.